A laminate formed by attaching a metal foil to a base material such as phenolic resin-impregnated paper and epoxy resin-impregnated glass fibers to a base material such as a film of polyimide or polyethylene terephthalate has been used for a printed circuit board substrate.
In this specification, a laminate for a printed circuit board substrate refers to a laminate prior to formation of circuits, a laminate whose metal foil is processed to form circuits is referred to as a printed circuit board, and both are referred to as a printed circuit board substrate.
In recent years, the area for wiring in the printed circuit boards used in the fields of electric and electronic instruments and precision instruments is becoming increasingly smaller and this situation has created a growing demand for multilayer printed circuit boards. A large variety of adhesives or adhesive films are used in fabrication of multiplayer printed circuit boards by lamination of printed circuit boards or in formation of composites from various kinds of circuit materials.
Adhesives in the form of prepregs prepared by impregnating fabrics such as glass fibers with epoxy resins or bismaleide-based resins are known for the aforementioned adhesives. However, the problem with them was their insufficient flexibility and inferior dimensional stability. Adhesives based on acrylonitrile butadiene rubber/phenolic resin, phenolic resin/butyral resin, and acrylonitrile butadiene rubber/epoxy resin have also been proposed (for example, in JPH4-29393 A, JPH4-36366 A, and JPH4-41581 A). However, these adhesives were not sufficient in chemical resistance, heat resistance, heat resistance after moisture absorption, and processability and they degrade markedly thermally and generate smears in drilling of through-holes.
Further, polyimide-based adhesives of excellent heat resistance have been proposed (for example, in U.S. Pat. No. 4,543,295). However, the polyimide used in such adhesives needs a temperature of 250° C. or above for heat contact bonding of copper to polyimide or polyimide to polyimide with a sufficient adhesive strength and this presents a difficulty in practical use.
The patent document 1 discloses an adhesive which is based on a polyimide resin prepared from a diaminopolysiloaxane and an aromatic tetracarboxylic acid as raw materials for heat contact bonding at low temperature. However, a polyimide resin of this kind alone has a defect of insufficient adhesive strength and poor reliability.
A polyimide-based adhesive of excellent adhesive strength is disclosed, for example, in the patent document 2; the adhesive film disclosed here is composed of a polyimideamide resin and an epoxy resin and intended for use as an adhesive in fabrication of flexible printed circuit boards (FPC). However, when this kind of film is used to bond printed circuit boards whose surface is made uneven as a result of formation of circuits in fabrication of multilayer printed circuit boards, the film is unable to fill the surface depressions of the circuit boards sufficiently and fails to provide sufficient solder heat resistance.
Under the circumstances, a demand has arisen for materials useful as adhesives for multilayer printed circuit board substrates and coverlay films with the emphasis laid on that the materials can be used in low temperature contact bonding of less than or equal to 250° C. and are excellent in adhesive strength, chemical resistance, solder heat resistance after moisture absorption, dimensional stability during wiring processing, and other properties. Furthermore, materials with an additional property of flame retardance are in demand from the viewpoint of securing safety from fire.
Resins or additives containing a halogen such as bromine have been used to provide conventional adhesive films with flame retardance. They have enjoyed universal use because of their ability to provide flame retardance, high cost performance, and the least tendency to degrade plastics. However, halogen-containing adhesives are feared to have a possibility of generating toxic substances such as dioxin when burned and removal of halogens is strongly demanded.
A variety of flame-retardant materials containing no halogen are being developed as substitutes for those containing halogens. The most general technique adopted in the developmental works is use of phosphorus-containing resins or addition of organic phosphorus compounds. Materials containing phosphorus compounds are disclosed, for example, in JP2004-231792 A, JP2005-15761 A, JP2005-171044 A, and JP2005-60489 A. However, phosphorus compounds are feared to have a possibility of contaminating soil and water and, in view of their present doubtful status regarding safety, it will become necessary to remove phosphorus compounds from flame-retardant materials in the future and adhesive films will be no exception.
In consideration of these matters, there is a demand for flame-retardant adhesive films which are free of halogens and phosphorus. Adhesive films of this type are described, for example, in the patent document 3. However, the adhesive films cited there had a problem in that their anti-migration properties deteriorate by the effect of components such as metal hydroxides.
The patent document 4 discloses a heat-resistant adhesive film which is composed of a silicone unit-containing polyimide resin and an epoxy resin and intended for printed circuit boards. However, the epoxy resin used here is an ordinary epoxy resin derived from bisphenol A or a phenolic resin and it appears that the kind of polyimide and a polyimide/epoxy resin combination are selected routinely without giving enough consideration to attain flame retardance. Further, the patent document 5 discloses an acenaphthylene-modified phenolic resin and an epoxy resin obtained by epoxidation of the naphthylene-modified phenolic resin. However, no disclosure is made on a possibility of combining the acenaphthylene-modified epoxy resin or any other epoxy resin with a polyimide resin to prepare an adhesive.    Patent document 1: JPH4-23879 A    Patent document 2: JPS52-91082 A    Patent document 3: JP2004-146286 A    Patent document 4: JP2001-203467 A    Patent document 5: WO2003/104295 pamphlet